Deuterium supersaturated surface layer in tungsten: ion energy dependence

Abstract

Properties of deuterium (D) supersaturated surface layers (DSSLs) formed in tungsten (W), such as thickness, internal microstructures, and D retention, are experimentally investigated as a function of the incident ion energy, E _i . W samples were exposed to D plasmas in the PISCES-A linear plasma device in a range of E _i ∼ 45–175 eV, while other plasma exposure parameters were fixed: sample temperature, T _s , ∼423 K, ion flux, Γ _i , ∼1.2 × 10 ^21 m ^−2 s ^−1 , and fluence, Φ _i , ∼3.0 × 10 ^24 m ^−2 . High-resolution, cross-sectional, transmission electron microscopy observations confirm that (1) a DSSL forms even at the lowest E _i ∼ 45 eV, (2) the DSSL thickness, Δ t _DSSL , is found to decrease with decreasing E _i from ∼11–12 nm at E _i ∼ 175 eV to ∼5–6 nm at ∼45 eV, and to agree with approximately the maximum implantation depth calculated using SDTrimSP, and (3) high-density D nanobubbles with a diameter of ∼1 nm or less exist inside the DSSL, which is deemed to validate a theory-predicted vacancy stabilization process due to trapping of a solute D atom(s). Utilizing a D areal density of ∼4.2 × 10 ^19 m ^−2 in the first 14 nm from the surface at E _i ∼ 75 eV from nuclear reaction analysis and the measured E _i dependence of Δ t _DSSL , our previous laser-induced breakdown spectroscopy data is updated: both dynamic and static D retention increase with decreasing E _i , and the D/W atomic fraction during plasma exposure reaches ∼0.3 at E _i ∼ 45 eV. A possible DSSL formation mechanism is proposed.

Keywords

• deuterium supersaturated surface layer
• tungsten
• nanobubbles
• FIB
• TEM
• NRA